DUBAI -- Saudi Arabia has detained two senior members of the Saudi royal family - Prince Ahmed bin Abdulaziz, the younger brother of King Salman, and Mohammed bin Nayef, the king’s nephew, two sources with knowledge of the matter said.

Crown Prince Mohammed bin Salman, King Salman’s son and the de facto ruler of the world’s top oil exporter and key U.S. ally, has moved to consolidate power since ousting his cousin, Mohammed bin Nayef, as heir to the throne in a palace coup in 2017.

He arrested several royals in an anti-corruption campaign later that year. 

One source said the detentions took place on Friday. Reuters could not immediately determine the reasons behind the detentions. 

The Wall Street Journal reported the detentions of the two royals earlier on Friday, and said they related to an alleged coup attempt. 

Saudi officials could not be immediately reached for comment early on Saturday.

The Saudi government media office did not immediately respond to a Reuters request for comment. 

Prince Mohammed has fueled resentment among some prominent branches of the ruling family by tightening his grip on power and some question his ability to lead following the 2018 murder of a prominent journalist by Saudi agents and the largest-ever attack on Saudi oil infrastructure last year, sources have said. 

They said royals seeking to change the line of succession view Prince Ahmed, King Salman’s only surviving full brother, as a possible choice who would have support of family members, the security apparatus, and some Western powers. 

Saudi insiders and Western diplomats say the family is unlikely to oppose the crown prince while the 84-year-old king remains alive, recognizing that the king is unlikely to turn against his favorite son.

The monarch has delegated most responsibilities of rule to his son but still presides over weekly cabinet meetings and receives foreign dignitaries. 

Prince Ahmed has largely kept a low profile since returning to Riyadh in October 2018 after two and a half months abroad.

During the trip, he appeared to criticize the Saudi leadership while responding to protesters outside a London residence chanting for the downfall of the Al Saud dynasty. 

He was one of only three people on the Allegiance Council, made up of the ruling Al Saud family’s senior members, who opposed Mohammed bin Salman becoming crown prince in 2017, sources have earlier said.

Mohammed bin Nayef’s movements have been restricted and monitored since then, sources have previously said. 

The latest detentions come at a time of heightened tension with regional rival Iran and as Crown Prince Mohammed implements ambitious social and economic reforms, including an initial public offering by oil giant Saudi Aramco on the domestic bourse last December.

Saudi Arabia is also the current chair for the Group of 20 major economies. 

Prince Mohammed has been lauded at home for easing social restrictions in the Muslim kingdom and opening up the economy.

But he has come under international criticism over a devastating war in Yemen, the murder of journalist Jamal Khashoggi in the kingdom’s Istanbul consulate and the detention of women’s rights activists seen as part of a crackdown on dissent.